The man was working as subcontractor for Dallas-based operator Energy Transfer Partners, which is building the nearly 1,200-mile pipeline from North Dakota to Illinois.

"We are saddened to learn that an employee of a subcontractor on the project in western North Dakota has died in an apparent work-related accident," the company said in a statement. "Our hearts and prayers are with his family. We do not have any additional details at this time."

The pipeline begins in western North Dakota and already is completed there, Kalk said. The site of the death is more than 200 miles away from where hundreds of mostly American Indians are protesting the pipeline in southern North Dakota. Tribal members fear the pipeline will harm water supplies and destroy sacred sites.

Kalk said the man was on a tractor Thursday, covering the underground pipeline with soil and grass seed. Kalk said the company reported Friday that the man suffered a serious head injury, apparently while working on equipment. He was taken to a Minot hospital, where he died.

The man was working alone and was found by his foreman, Kalk said.

"The company no reason to believe this was anything other than terrible accident," Kalk said.

State and federal regulators expect a full report on the incident early next week, he said.

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